The present invention relates to a new and improved construction of speed limiting device for lifts or the like of the type comprising a cable wheel drivable by a limiter cable secured to the lift cabin, a cam disk fixedly connected with the cable wheel and seated therewith upon a common shaft or axle and a support movable by the cam disk and wherein the speed limiting device, when there is exceeded a predetermined lift travel speed, decelerating or retarding the limiter cable.
Passenger lifts and freight lifts used by individuals must be equipped with a catch mechanism which is triggered by a speed limiter, mounted at the upper end of the lift chute or shaft and driven at a rotational speed proportional to the speed of travel of the lift cabin, upon exceeding the normal travel speed by a predetermined value.
The heretofore known speed limiters or speed limiting devices are classified into two main groups, the pendulum limiters and the centrifugal limiters.
With the state-of-the-art pendulum limiters a pendulum lever which is subjected to the pressure of a spring is placed into oscillatory motion by a revolving cam of a cam disk which is fixedly connected with a cable wheel, and depending upon the shape of the cam there is imparted to the pendulum lever certain accelerations. With impermissible speed of the lift the pendulum lever swings-out a greater extent and thus causes a pawl which is fixedly connected therewith to come into engagement with the teeth of a blocking wheel which is likewise fixedly mounted at the cable wheel, so that the speed limiter is instantaneously blocked. The drive cable which is trained about the cable wheel of the speed limiter is thus braked and the catch mechanism connected with the drive cable and arranged at the lift cabin is released or triggered, wherein depending upon the construction of the catch mechanism the lift cabin is brought to standstill with a more or less pronounced deceleration.
Although the state-of-the-art pendulum limiters, on the one hand, constitute relatively inexpensive robust constructions which embody only very few moved components, on the other hand they are, however, associated with a number of decisive drawbacks. In particular, for constructional reasons it is not possible to exceed a certain spacing of the cams or teeth of the cam disk or the blocking wheel, respectively. Hence, there can exist between the response time of the speed limiter and the release time of the catch mechanism a difference during which the speed of the lift cabin is accelerated to impermissible values. Furthermore, when reaching a critical speed there can arise an unstable condition wherein the accelerating force acting upon the pendulum lever is equal to the opposing spring force, with the result that there can arise damage to the pawl and the teeth. Consequently, there can arise an overshooting of the cam and teeth, producing a further increase of the difference between the response time of the pendulum limiter and the release time of the catch mechanism. Additionally, due to lifting-off of the roller rotatably mounted at the pendulum lever from the cam disk there is no longer insured for a faultless functioning of the pendulum limiter at greater speeds or accelerations. Further drawbacks with this type of speed limiter can be enumerated in terms of the occurrence of fluttering or chattering noises and the need to install a slipping or release clutch, since the elongation of the cable which occurs due to the sudden standstill of the pendulum limiter must be maintained within permissible limits.
In order to prevent triggering or release of the catch mechanism in the presence of small speed excesses, a speed limiter can be equipped with an electric pre-cutoff. This electric pre-cutoff causes the lift control to be interrupted and brings the mechanical brake of the lift cabin to standstill.
According to a known apparatus at a pendulum limiter a mass body movably guided in vertical direction and exposed to the action of an adjustable compression spring is pressed against a stop and is operatively connected with a pendulum lever. When reaching a predetermined travel speed the acceleration forces raise the mass body off the stop against the force of the compression spring, and an actuation bolt fixedly connected with the mass body interrupts an electrical contact.
The drawbacks associated with the oscillating pendulum lever of a pendulum speed limiter also act unfavorably upon the pre-cutoff. An exact defined cut-off is rendered more difficult owing to the complicated dynamic behavior in conjunction with the dimensioning of the components of the pre-cutoff.
Generally, the centrifugal limiters or centrifugal speed limiters do not exhibit the drawbacks associated with the pendulum limiters or pendulum speed limiters noted above. They can be used throughout a large speed range and operate particularly in a faultless manner at high speeds. According to a state-of-the-art centrifugal limiter equipped with an electrical pre-cutoff two weights are pivotably mounted at the cable wheel or rope pulley. Secured to the weights are resilient elements which oppose the movements of the weights brought about by the centrifugal force during rotation of the cable wheel. At one of the weights there is articulated a rod which through the agency of an angle lever axially displaces a piston mounted in a central bore of the cable wheel hub. An initial small displacement of predetermined magnitude is transmitted through the agency of a further angle lever and a further rod directly to an electrical switch which initiates the pre-cutoff and the regulation of the speed. During a greater axial displacement of the piston a pawl is released, resulting in actuation of the movable jaw of a cable brake and the initiation of the braking operation.
The drawbacks of such centrifugal limiters especially reside in their complicated construction, requiring a relatively large number of components. Consequently, it is practically impossible to realize an economical solution. A further disadvantage resides in the fact that the centrifugal limiter is not capable of functioning in a faultless manner at lower speeds.